How to Build a Simple Pac Man Game - Scratch

[Pages:13]How to Build a Simple Pac-Man Game

For today's program, we are going to build a simple Pac-Man game. Pac-Man was one of the very first arcade games developed around 1980. For our version of Pac-Man we are going to focus on the following programming and problem solving techniques:

a. Using Scratch to develop computer objects (also known as Object Oriented Programming classes) that will interact with each other in the game. We will only develop five sprites with scripts for this program (7 in total for the game) but will be using copies of the sprites to make a complete fully interactive video game (inheritance principle). Object Oriented Programming is a complex topic and this program will not use design patterns which are a hallmark of OOP. Your next project, the Marble Roll Game, will start to explore this area however.

b. Developing simple animation of characters in the game. Animation code (blocks) will be kept separate from functionality code (blocks) so that sprites can be reused with simple changes to the costumes and minor changes to some variables. This is how computer languages and scripts that are fully object oriented (such as Action Script 3 for Flash or Java) work.

This lesson is designed for intermediate to advanced users of Scratch who have had some experience in working with the software and developing Scratch applications. Basic concepts such as how to create new sprites, where to write scripts, etc. will not be reviewed. The software in this series of lessons only covers creating a single level for this version of Pac-Man, however, the software has been written so that multiple levels are easy to add with simple changes to variables. We will discuss how to do this briefly at the end of the lesson.

Getting Started To get started creating our PacMan program, we are going to build a total of seven sprites. Start by creating a sprite named PacMan with only a single costume.

Then create a sprite called Horizontal to be a horizontal bar and Vertical which will be a vertical bar. The sprites will be used to build the maze for Pac-Man and should both be the same color. Next we will create a sprite called Point Pill that Pac-Man will eat as he goes through the maze. We will then create a sprite called Ghost 1 with a single costume. This ghost will be Pac-Man's enemy and will be copied to make further ghosts later in the game. Lastly, we will create a sprite called PowerUp in any color that is different from the Point Pill sprite. When PacMan eats a PowerUp, he will have the ability to eat the ghosts for about 10 seconds and score 100 points for every ghost he eats. Our last sprite will be called GameOver and will display a game over message on screen once three Pac-Man characters have been eaten by the ghosts and the game is finished.

Next we will create a total of eight variables for our game. Our first variable will be called Chase Pac-Man. This variable will be used to determine if Pac-Man can eat the ghost or if the ghost can eat Pac-Man. Our next variable will be called Direction. The Direction variable will be used to randomly determine which directions the ghosts travel on the screen (up, down, left, or right). Each ghost needs to have its own direction variable. In our example we will create a total of four ghosts. Thus Direction Ghost 2, Direction Ghost 3, and Direction Ghost 4 are used so that all ghosts can travel separate directions from each other. The next variable we will create is called Ghost Speed. Ghost speed will be used to determine how fast the ghost will travel and can be increased with each level of the game to make the game more challenging. Our next variable will be called Lives. Lives will be used by Pac-Man to determine when the game is over once he has been eaten three times by the ghosts. Our last variable is called score and will be checked so that it appears on the screen. This is used to keep track of the score in the game. With our initial sprites and variables created let's turn to making our first character the Pac-Man.

PacMan Set-up

Let's start by setting our initial condition for Pac-Man. The first thing we need to decide is how our character will move on the screen. To simplify the animation of Pac-Man the first thing I'm going to do is select the can rotate button to control his direction on the screen. This gives us the ability to easily allow PacMan to turn and move in four directions. The disadvantage to this selection is that Pac-Man's character will appear upside down when he is going left on the screen as selecting this flips the character in the opposite direction. To get around this we will use some creative programming of our costumes when Pac-Man is facing this direction. This will be explained later in the directions.

Next we will create a total of seven different costumes. One of the costumes must have Pac-Man's mouth closed as this will be the default costume. The next three costumes will show his mouth in various states of being open. By flipping through these costumes we will add animation to Pac-Man and make him appear to eat. For costumes numbers 5 to 7 we will flip him upside down, again, with his mouth being open in various states. When our default costume is combined with these upside down costumes and Pac-Man is facing to the left, he will be rotated around (rather than being upside down) and appear to be eating while going in the left direction. This will be further explained when we look at the animation blocks. Feel free to add more or less costumes to your Pac-Man, however, you will need to adjust the number of blocks when we get to the animation scripts.

use of a control block to move him.

PacMan Main Script

The main Pac-Man script is used to set Pac-Man up in his initial location and allow him to move around the screen. It also determines what happens when Pac-Man hits the wall the maze or the edge of the screen. When the green arrow is clicked the variable Lives are set to zero (meaning he now has three lives) he is rotated to 90? (his) starting direction, moved to his initial spot on the screen and is shown. The Pac-Man script then enters a forever loop that will check to see if keys are being pressed that would allow him to move.

First, it checks to see if the right arrow key is being pressed. If it is, it rotates Pac-Man around facing the right direction (90?) and moves him 3 steps to the right. If however, he bumps into a maze (or he is touching a maze color) it pushes him away from the maze wall in the opposite direction so we cannot move through the maze. The rest of the if statements in this loop do similar things checking to see if Pac-Man should move to the left, up, or down and moving in that direction unless he hits the maze wall in which case he is stopped. At the bottom of the loop if he is touching the edge he simply bounces back onto the screen. By controlling his movement through the use of a forever block, the Pac-Man character is much more responsive to key presses then he would be through the

Pacman Animation Script Animation for the Pac-Man game is kept separate from the control loop. This serves two purposes. First, it makes our sprites more reusable. Sprites (objects) can be copied and reused and simple changes can now be made to costumes and animations with out changing the interactivity of the sprite. This allows us to create multiple characters quickly and easily. Second, the animation can be made smoother as it does not need to wait for its "turn" in the sprites control loop. When the green flag is clicked a forever loop controlling and the animation is entered. The first part of the loop switches between our costumes 1 to 4 with a brief pause in between creating the illusion that the PacMan's mouth is opening and closing. At the bottom of the loop it checks to see if the direction of the Pac-Man is greater than -90 if it isn't then it switches to the second set of costumes (the upside down Pac-Man) and loops through these (repeats) until it is greater than 90. Let's look at how this works a bit further using a little bit of math. Scratch sees the direction of sprites as being between positive 180? and -180?. The top of a sprite is 0? while the bottom of the sprite is 180?. A character rotated to the right is a positive rotation will character rotated left is in negative rotation. Earlier in the pro-

gram we allowed Pac-Man to rotate as he's moved around the screen with the arrow keys. When the up arrow key is pressed Pac-Man's rotation is set to 0?. When the right arrow key is pressed his rotation is set to 90? and when the down arrow key is pressed its rotation is set to 180?. When the left arrow key is pressed its rotation is set to -90?. This results in the character being turned upside down. However in this loop when the character set to 90? the upside down costumes are animated then rotated around so they now face in the correct direction and the animation (for the most part) looks correct. There can be a slight delay between the control loop and the animation loop causing the animation to look upside down for a brief second. This is difficult to program around in the Scratch environment.

Pacman Eaten (by the Ghost) Script The Pac-Man sprite also contains a script that decides what happens when the Pac-Man is touched by a ghost when the power up is not active. The ghosts send a broadcast to the Pac-Man if they touch him when he is not powered up. The Pac-Man receives this message called a "Scare PacMan" in this script. The Pac-Man is hidden and plays a sound to show that he has been eaten by the ghost. If his Lives are greater or equal to three (meaning he has been killed three times already) it sends a broadcast called game over which will end the game (will look at this later). If this is not the case and more lives are available it will wait 2 seconds change the Lives by one (meaning takes one of his lives away) switches

him to the start costume for the program, places him back in the starting position, points him in the correct direction and then shows him on the screen and continues the game.

Point Pill Script The point pill script is used to have pills the Pac-Man can eat and score points on as he goes around the screen. When the green flag is clicked the pill is shown on the screen. It then enters a forever loop which checks to see if the pill is touching the Pac-Man. If it is, it plays a sound, changes the score by one (adding point to the Pac-Man score) hides the pill and then stops all scripts. The stopping of the scripts speeds up other animations in the software. If you're creating multilevel game you may just wish to hide the point pill and re-show it at a new level. This will be discussed further at the end of this tutorial.

Power Up Script The power up script allows the Pac-Man to eat the ghosts for a limited amount of time once he has eaten the power up pill. When the green flag is clicked the power up pill is shown on the screen and enters a forever loop. It checks to see if the pill is touching the Pac-Man. If it is, it broadcasts a power up message (that is received by the ghosts ) hides the pill that has been eaten by the Pac-Man and stops the pills script speeding up the software. Again if you're creating a multilevel game you may just wish to hide the PowerUp pill and re-show it at a new level. This will be discussed further at the end of this tutorial.

Game Over Script The game over script is simply a way to indicate to the user that they have lost the game. Create a sprite that simply says "Game Over". When the green flag is clicked hide this sprite. In the Pac-Man sprite we had it broadcast a message called "Game Over" when it lost more than three lives. That message is received by the Game Over sprite. When it receives a game over it brings the game over sprite to the front, shows it, and then stops all scripts in the program. This ends the game and the user must reclick the green flag in order to start the game again.

Main Ghost Script The main group ghost script controls the movement and interactivity of the ghosts in the program.

When the green flag is clicked a variable called Chase Pac-Man is set to 1 (meaning the ghosts are able to eat Pac-Man) and the ghost speed variable is set to 2. The ghost speed variable determines how quickly the ghosts move and can be increased (such as in different levels) to make the game more difficult. The ghost is then set to its initial position point in the correct direction and shown on the screen. A random number between one and four is then selected and given to the direction variable. This random number will determine the direction the ghost travels until it hits a wall and as such, each ghost will need it own direction variable (which we set up earlier) so they can operate in different directions from each other.

The program then enters a forever loop and checks to see which direction the ghost was set in. If the direction variable is 1 the ghost goes right, 2 the ghost goes left, 3 the ghost goes up, or 4 the ghost goes down. This is controlled by a bunch of if statements that checks to see what the direction variable is set to points the ghost in that direction and then changes its X or Y direction by the ghost speed variable which determines the speed of the ghost. If the ghost hits either the edge of the stage or part of the wall it is told to bounce away from the object and then pick a new

random number. When it goes back up to the loop it will have a new direction which it will follow until it again hits the maze or the edge. At the bottom of the main loop there is an if statement that checks to see if it's touching the Pac-Man and it's Chase Pac-Man variable is not = 0 (meaning the Pac-Man is not powered up to eat the ghosts). If this is the case the ghost broadcasts a "Scare PacMan" message which is received by the Pac-Man character costing the Pac-Man a life in the game.

Ghost Costumes & Rotation We will use the same rotation and costume tricks for the ghosts as we did for the Pac-Man to aid in animation and travel direction. Start by creating two ghost costumes, one that is upright and one that is upside down. Then selected the can rotate button up at the top of the screen. Now we'll create a costume script for the ghost. When the green flag is clicked enter forever loop that will switch to costume number one until the direction of the ghost is greater than -90?. When this happens the upside down costume (2) will be selected, rotated around to be the correct direction, and repeat like this until the direction of the ghost is greater than 90?.

Ghost Power Up Script The ghosts power up script allows the Pac-Man to eat the ghost when this script is running. When the Pac-Man eats a Power Up Pill it sends a message called PowerUP that is received by the ghost. The ghost sets the Chase Pac-Man to zero (meaning that the ghost can't eat Pac-Man), sets the Ghost Speed to 4 (meaning the ghost runs a little faster) and set the timer in the game to zero. The ghost then enters a repeat loop that continues until the timer is greater than 10, meaning 10 seconds have gone by. The ghost then has a color change meaning it flashes while the PacMan is able to eat it and checks to see if the Pac-Man is touching the ghost. If it is, it hides the ghost plays a sound to show it's been eaten, adds 100 points the PacMan score points it in the starting direction and moves the ghost back to its starting base in again. At the end of 10 seconds the repeat loop ends, sets the Chase Pac-Man back to one (mean the ghost can now eat the Pac-Man) sets the ghost speed back to normal, stops the flashing of the ghost and make sure it shows it on the screen again. If the Pac-Man had eaten the ghost it will reappear at this point, if it had not, it will simply stop flashing and return to its regular color.

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